With the recent widespread use of cordless equipment such as a personal computer or portable telephone, secondary batteries used as a power source of the cordless equipment are increasingly required to have a smaller size and higher capacity. At present, a lithium ion secondary battery that can achieve a small size, light weight, and high energy density is being put to practical use and growing in demand as a portable power source. However, depending on the type of cordless equipment to be used, the lithium ion secondary battery is not yet reliable enough to ensure a continuous available time.
Under these circumstances, a polymer electrolyte fuel cell has been studied as an example of the battery that may meet the above requirements. The polymer electrolyte fuel cell uses a polymer electrolyte membrane as an electrolyte, oxygen in the air as a positive active material, and a fuel (hydrogen, methanol, etc.) as a negative active material, and has attracted considerable attention because it is a battery system that can be expected to have a higher energy density than the lithium ion secondary battery. Fuel cells can be used continuously as long as a fuel and oxygen are supplied. Although there are several candidates for fuels used for the fuel cells, the individual fuels have various problems, and the final decision has not been made yet.
A direct methanol fuel cell (DMFC) is miniaturized easily and expected to be a future portable power source. In the DMFC, methanol is used as a fuel and reacts directly at the electrode. However, the DMFC causes a reduction in voltage due to a crossover phenomenon in which methanol at the negative electrode passes through the solid electrolyte and reaches the positive electrode. Therefore, the DMFC still does not have the expected energy density.
When hydrogen is used as a fuel, e.g., a method for supplying hydrogen stored in a high-pressure tank or hydrogen-storing alloy tank is employed to some extent. However, a fuel cell using such a tank is not suitable for a portable power source, since both the volume and the weight of the fuel cell are increased, and the energy density is reduced. There is also another method for extracting hydrogen by reforming a hydrocarbon fuel. However, a fuel cell using the hydrocarbon fuel requires a reformer and thus poses problems such as the supply of heat to the reformer and the thermal insulation. Therefore, this fuel cell is not suitable for a portable power source either.
Under these circumstances, a method has been proposed that produces hydrogen with a chemical reaction at a low temperature of 100° C. or less and uses the hydrogen as a fuel. For example, a metal that reacts with water to produce hydrogen such as aluminum, magnesium, silicon, or zinc is used as a hydrogen source (see the following Patent Documents 1 to 5).
Patent Document 1: U.S. Pat. No. 6,506,360
Patent Document 2: JP 1(1989)-61301 A (U.S. Pat. No. 2,566,248)
Patent Document 3: JP 2004-231466 A
Patent Document 4: JP 2001-31401 A
Patent Document 5: JP 2004-505879 A
Patent Documents 1 to 3 disclose methods including the reaction of aluminum and an alkali or acid. Although it may be easy for these methods to produce hydrogen chemically, the equivalent weight of the alkali or acid corresponding to aluminum needs to be added, which in turn reduces the energy density because of a large proportion of the material other than the hydrogen source. Moreover, the reaction product (oxide or hydroxide) forms a film on the surface of the metal, so that water cannot come into contact with the inside of the metal. This may lead to a problem that the oxidation reaction stops only at the surface of the metal. In particular, it is difficult for the method of Patent Document 3, in which heat of the reaction between calcium oxide and water is utilized in the hydrogen producing reaction of aluminum, to generate hydrogen if the content of the calcium oxide is less than 15 wt %. In Patent Document 3, therefore, the proportion of aluminum in the hydrogen generating material is 85 wt % or less.
Patent Document 4 is intended to avoid the above problem by removing the film mechanically from the metal surface. However, the device should have mechanical equipment for removal of the film and becomes larger. In Patent Document 5, alumina is added as a catalyst to suppress the formation of the hydroxide film, and hydrogen is generated at a low temperature of 50° C. However, the addition of a certain amount of catalyst can reduce the content of aluminum in the hydrogen generating material.